1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Discussion of the Background
Recently, the development of the information processing systems using electrophotography has been significant. Among these, optical printers, which convert information into digital signals to optically record the information, have been extremely improved in terms of the quality of printing and reliability. This digital recording technology is applied to not only printers but also typical photocopiers, which leads to the development of digital photocopiers. In addition, it is anticipated that a typical analogue photocopier using this digital recording technology is more and more demanded because such a photocopier has various kinds of information processing functions. Further, with the diffusion and the improvement of performance of home computers, the development of a digital color printer to output color images and documents increasingly speeds up.
Negative-positive development is adopted in most of such digital image forming apparatus. This is because most of the images (originals) output by a typical image forming apparatus are characters (letters) so that the writing ratio based on the total area of an output sheet (image) is relatively low, i.e., from 5 to 10%.
The conversion from analogue to digital has an aspect that the positive-positive development is converted into the negative-positive development. The significant difference therebetween is a change from irradiating an entire original with light and thereafter an image bearing member with the resultant light to writing only information of letters, etc. on an image bearing member. This has an advantage in that since the information portion of an original is low, i.e., not greater than 10%, the output time of a light source can be reduced to about not greater than a tenth, which leads to elongation of life of a writing light source.
In an image forming apparatus using the negative-positive development, charges remain in the portion (non-image portion, background portion) on the surface of an image bearing member where writing is not performed by a writing light source. Since these remaining charges may affect the performance of the next charging process, a discharging process is introduced. There are methods of discharging, for example, an optical discharging method in which a photocarriers generated by light erase the surface remaining charges, a method in which an electroconductive brush is brought into contact with an image bearing member to leak the surface remaining charges, and a method in which a reverse bias is applied to the surface of an image bearing member to cancel the surface remaining charges.
With the improvement on the quality of image and the advance of colorization, the image forming apparatuses of late can perform writing and developing with a relatively high definition and the information (originals) input on the image forming apparatus is somewhat changing. As described above, letters used to be main images in originals but now photographs, colorized pictures, and graphs are inserted these days. In an original in which various kinds of patterns are input as signals, when the history of the image formed just one cycle before is not completely erased, the newly formed image may be affected thereby. In most case, this results in production of an abnormal image referred to as the residual image. When a residual image is produced, the image bearing member is already non-uniformly charged before the charging process starts. When a writing is uniformly performed in a particular area of the image bearing member, the charging voltage should be almost uniform but due to the history of the image formed one cycle before, the actual charging voltage is non-uniform. This happens especially when gradation is written. When a latent electrostatic image is formed in a state in which the charging voltage is non-uniform and developed by a developing device, the development is not correctly performed and the obtained image seems as if the image formed just one cycle before reflects.
There are two reasons for this. One is that, as image formation is performed at an extremely high speed, the capacity (charging ability) of a charging device tends to be insufficient. Therefore, when the surface charges on an image bearing member is not uniform before charging, this charged state is reflected in the next charging process, which results in non-uniformity of charging. The other is that, due to the size reduction of an image forming apparatus, colorization and the onset of a tandem type, a charging roller is used more frequently. A charging roller charges the surface of an image bearing member by the difference of the bias between the image bearing member and the charging roller caused by discharging therebetween. However, when the surface charges on an image bearing member is not uniform before charging, this history is left in the next charging.
Therefore, how to make the surface voltage (surface charges) of an image bearing member before charging uniform is the key. Consequently, the discharging process is a significant process to improve the quality of images.
The discharging methods mentioned above except for the optical discharging method have the following drawbacks. The method using an electroconductive brush requires a member contacting the surface of an image bearing member. Therefore, the abrasion between the contacting member and the image bearing member is inevitable and has an adverse impact on the durability of the member and the image bearing member. In addition, the effect of leaking charges may be reduced when the surface of an image bearing member and of the contacting member is contaminated. Furthermore, considering the transfer time of the charges, the leaking method is not suitable for high speed printing.
In the case of the method in which a reversed bias is applied, when the bias condition is insufficient, the remaining charges on the surface of an image bearing member is not sufficiently cancelled. To the contrary, when the bias is applied too much, the surface of an image bearing member is reversibly charged. As to a typical image bearing member, only positive charges can be transferred. Therefore, when the surface of an image bearing member is positively charged, the positive charges are not cancelled. Therefore, for next charging, these positive charges are cancelled first and thereafter, the image bearing member is negatively charged. This accelerates the deficiency of the capacity of a charging device. In addition, when the surface of an image bearing member is positively charged, traps are easily generated in a photosensitive layer and the residual voltage of the image bearing member tends to be generated. As a result, the life of the image bearing member is shortened.
Consequently, the optical discharging method is now the best selection as the discharging process for use in an image forming apparatus.
As described above, the image forming apparatus adopting a digital system has become prevalent recently, in which the negative-positive development is adopted in terms of what is described above. Therefore, the significance of the discharging in the negative-positive development is relatively large in comparison with the positive-positive development taken by the analogue system.
That is, in the positive-positive development, when optical discharging is performed after transfer, the entire of an image bearing member is irradiated with light but actually the area corresponding to the portion on which letters, etc., are written is discharged. The area is 10% at best based on the total area of the surface of the image bearing member.
In contrast, in the negative-positive development, the non-irradiated portion (where writing is not performed) is actually discharged. This means, when the same original as in the case of the positive-positive development is used, almost 90% of the total surface area of an image bearing member is actually discharged. “Actually discharged” means that the image bearing member has surface charges in the optically discharged area and the surface charges are erased by photocarriers generated in the image bearing member by irradiation of light by a discharging device.
This means that, in the typically used positive-positive development and the negative-positive development, the ratio of the surface charges of an image bearing member which are erased is totally different (reversed). This difference is about 10 times with regard to discharging. However, image bearing members have been used as they are without studying the influence of the discharging process on an image bearing member significantly.
As technologies with regard to discharging devices, unexamined published Japanese patent applications Nos. (hereinafter referred to as JOP) S60-88981 and S62-87981 describe that optical fatigue and charging fatigue can be prevented by irradiation of suitable discharging light containing light having a short wavelength in the case of an image forming apparatus using an inorganic image bearing member (Se based or a-Si). The image bearing members for use in JOP S60-88981 and S62-87981 are inorganic image bearing members, which have an essentially different photocarrier generating mechanism from those of an image bearing member using an organic charge generating material, which is described later. The technologies applicable to an inorganic based optical discharging may not be applicable to an organic based optical discharging. The development used in S60-88981 and S62-87981 is the positive-positive development, meaning that the discharging does not have the same weight as for the negative-positive development. Furthermore, the discharging light contains light having a wavelength of not less than 500 nm. As the result of the experiment using the technologies described in S60-88981 and S62-87981, it is found that the restraint effect on the rise of the residual voltage is not sufficient.
JOP S61-36784 describes that the residual voltage can be removed by discharging with light having a wavelength which almost matches the specific photosensitive wavelength of a dye-sensitized image bearing member. In other words, discharging is performed using light having a wavelength in the range in which the light is not absorbed by the sensitizing dye. However, the original material (e.g., polyvinyl carbazole) of the dye-sensitized image bearing member does not absorb visible light so that a sensitizing dye is used which can absorb visible light. Therefore, in the technology, discharging is performed with light having a wavelength in the range in which the light is not absorbed by the dye but polyvinyl carbazole. In this wavelength range, photocarriers are not effectively generated so that discharging is not effectively performed and polyvinyl carbazole is optical fatigued. Therefore, this technology is not effective in some cases. In addition, also the development method is the positive-positive development. The meaning of discharging is totally different from the case in which the negative-positive development is used. Therefore, when this technology is applied to a negative-positive development case, which is the target of the present invention, the effect is not sufficient.
JOP S62-38491 describes that the deterioration of the sensitivity ascribable to the optical fatigue of an image bearing member having a photosensitivity on the long wavelength side and a low or practically no photosensitivity on the short wavelength side can be restrained by discharging the image bearing member with light having a short wavelength. However, when a relatively high speed image forming apparatus is used and discharging is performed with light which is low or practically not sensitive, the discharging performance is not sufficient and abnormal images are produced in some cases. It is desired that an image bearing member has sufficient photosensitivity to discharging light. The technology described in JOP S62-38491 is not possible to deal with the drawback mentioned above for the current image forming apparatus. Furthermore, the technology described in JOP S62-38491 is not specific with regard to the wavelength of discharging light.
JOPs H01-217490 and H01-274186 describe that the residual voltage is reduced by discharging by irradiating a positive charging type image bearing member in which a charge generating layer is accumulated on a charge transport layer with light having a wavelength of not greater than 620 nm. The discharging light contains light having a wavelength of not less than 500 nm. It is found from the result of the experiment in which technologies of JOPs 1-101-217490 and 1-101-274186 are applied that the rise of the residual voltage is not sufficiently restrained.
JOP H04-174489 describes that the rise of the residual voltage is restrained in an environment of a high temperature and a high humidity by discharging by irradiation using two kinds of luminous diodes simultaneously. The discharging light contains light having a wavelength of not less than 500 nm. It is found from the result of the experiment in which the technology of JOP H04-174489 is applied that the rise of the residual voltage is not sufficiently restrained.
Japanese patent No. (hereinafter referred to as JP) 3460285 describes that discharging is effectively performed by irradiating a single layered image bearing member containing an organic pigment with light containing a wavelength of not less than the half value width of the maximum absorption of light of the photosensitive layer. In general, a practically usable organic pigment has an absorption peak in the visible light range. Therefore, the technology of JP 3460285 is not sufficient to restrain the rise of the residual voltage.
JOP 2002-287382 describes that the residual voltage can be reduced and the occurrence of ghost can be restrained by discharging by selecting the wavelength where the sensitivity of an image bearing member for discharging light is higher than the sensitivity thereof for writing light. In this technology, the discharging wavelength varies depending on the material for use in an image bearing member. Therefore, it is impossible to specify the wavelength of discharging light. In addition, in general, an organic pigment has a light absorption peak in the visible light range. Thereby, its spectroscopy sensitivity peak also exists in the visible light range. Thereby, the technology of JOP 2002-287382 cannot produce a discharging technology using light having a wavelength shorter than 500 nm.
JOP 2005-31110 describes that, in discharging of an image forming apparatus using a charge generating material dispersion type (single layered) image bearing member, irradiation of light having wavelength having a relatively low spectroscopic absorption of the image bearing member is performed. This technology is used to remove the residual charges generated in a photosensitive layer bulk over repetitive use of a single layered image bearing member. That is, in the case of a single layered image bearing member, the charge generating material is uniformly dispersed in the bulk and light having a wavelength which has a high absorption ratio is absorbed around the surface of the photosensitive layer. This phenomenon is used for improving the definition of a single layered image bearing member. The writing light has a wavelength which is largely absorbed in a photosensitive layer to reduce the energy of irradiation and to generate photocarriers only near the surface of the photosensitive layer. Thereby, the transfer distance of charges having a polarity reverse to the polarity of the charging of the surface of an image bearing member is small, which prevents diffusion caused by Coulomb repulsion. On the other hand, the irradiation light hardly reaches the inside of the photosensitive layer bulk. Therefore, charge having the same polarity as the charges on the surface of the image bearing member tend to be trapped in the photosensitive layer bulk and may raise the residual voltage. It is impossible to cancel these charges. Considering this, light having a wavelength which can reach the bulk in depth is used as the discharging light to generate photocarriers at deep positions (close to the electric pole or the center of the photosensitive layer) in the photosensitive layer bulk, thereby canceling these charges.
To the contrary, different from a single layered photosensitive layer, the charge generating layer in a layered image bearing member is relatively extremely thin. In addition, the ratio of absorption of light is not greater than 90% even at the maximum absorption wavelength (in other words, at least 10% of writing light passes through the charge generating layer). This means, the generation of charges still occurs in the entire photosensitive layer bulk. Consequently, the effect described in JOP 2005-31110 is not true in the case of a layered image bearing member.
JOP 2004-45996 describes that discharging is performed by light irradiation to the soret band of a phthalocyanine compound and a fluorescent lamp is used as the discharging light source.
Furthermore, JOP 2004-45997 describes that discharging is performed for an image bearing member containing a phthalocyanine compound by using a fluorescent lamp.
FIG. 1 is a spectroscopic luminescent spectrum of light of a fluorescent lamp. A fluorescent lamp includes bright lines and in addition a large luminescence in the range of from 500 to 650 nm. The amount of irradiation of light, i.e., the amount of luminescent amount, depends on the area of FIG. 1. Therefore, the light having the wavelength range (from 500 to 650 nm) is mainly used as the light of a fluorescent lamp with which an image bearing member is irradiated.
In JOPs 2004-45996 and 2004-45997, discharging using a red LED light having a wavelength of 680 nm is compared with discharging using a fluorescent lamp. Judging from the spectrum of FIG. 1, the comparison is made between discharging using a red LED light having a wavelength of 680 nm and discharging by luminescence in the wavelength range of from 500 to 650 nm. Since the light corresponding to the soret band of a phthalocyanine compound is not zero, it is true that there is irradiation of light to the soret band. However, the composition of light occupies more than the composition corresponding to the soret band, which means the absorption in the area other than the soret band is large.
JOP 2005-181991 describes an image forming apparatus in which all of writing light wavelength (λa), discharging light wavelength (λb), and the maximum spectroscopic sensitivity wavelength (λc) are in the range of from 380 to 520 nm and the following relationship: λa<λb<λc is satisfied. Optical discharging by light having a wavelength in the blue light range is common in the technology described in JOP 2005-181991 and the present invention. However, it is impossible for the image bearing member (intermediate layer) of JOP 2005-181991 to absorb the discharging light so that the effect of the present invention is not expected.
In these situations, improvement on the quality of images and the durability, including colorization, is demanded for the printer and the photocopier mentioned above. There are two issues for the improvement on the quality of images for a digital apparatus. One is how to uniformly form a latent electrostatic image with fine dots. The other is how to reduce the occurrence of various kinds of abnormal images. In addition, with regard to the improvement on durability, it is highly effective to elongate the life of an image bearing member.
There are various kinds of approaches to solve these issues. The thing common in both issues is how to restrain the deterioration of an image bearing member caused by electrostatic fatigue for use in these image forming apparatuses. To be specific, it is how to restrain the rise of the residual voltage (voltage at irradiated portions) during repetitive use.
To restrain the rise of the residual voltage, the design (composition, structure, etc.) of an image bearing member has been devised in the development so far. However, the electrostatic fatigue of an image bearing member greatly depends on the compositions thereof and the process conditions. From a point of developers of an image bearing member, the developers are required to deal with each process condition. Under these circumstances, the study on the electrostatic fatigue of an image bearing member based on the process conditions have hardly been made.